1. Field of the Invention
The present invention relates to a method of producing ferromagnetic metal powder, especially composed mainly of iron, and more particularly to a method of producing ferromagnetic metal powder usable as a high density magnetic recording medium for a magnetic tape, a magnetic drum, magnetic disc or the like.
2. Brief Description of the Prior Art
Many methods of producing ferromagnetic metal powder have been investigated heretofore. These methods can be roughly classified as follows:
I. method comprising the steps of dissolving a compound of ferromagnetic metal into water and adding thereto reducing agent containing tetrahydride borate ions or hypophosphite ions, PA1 Ii. method comprising the step of reducing an oxalate, a formate, a oxide or a oxyhydroxide of ferromagnetic metal in a reducing atmosphere at an elevated temperature, PA1 Iii. method comprising the steps of preparing an aqueous solution in which a compound of ferromagnetic metal is dissolved and electrolyzing the solution, and so on.
The metal powder for a magnetic recording medium of high density is desired to be composed of particles of high coercive force and high remanence ratio having needlelike shapes and uniform size. However, it is difficult to obtain such needlelike shaped and uniform-sized metal powder by the above-mentioned prior art method. Namely, according to the above-mentioned prior art method (i), we can obtain a powder composed of only sphere-shaped particles, dielike-shaped particles or chain-sphere-shaped particles which are formed by linking the globe-like or dielike-shaped particles together. According to the above-mentioned prior art method (iii), the shape of the obtained particles of the powder is only dendritic. Furthermore, in the above-mentioned prior art (ii), although somewhat needlelike shaped particle powder can be obtained when the needlelike shaped oxalate, formate, oxide or oxyhydroxide is used as starting material, it is difficult to obtain uniform-sized metal powder having needlelike shapes because the particles link together or break in the reducing process at an elevated temperature. As mentioned above, it is very difficult to obtain needle-like shaped ferromagnetic powder according to the prior art.